The present invention relates to a rotary trimmer potentiometer having a gear mechanism for the fine adjustment of the resistance and, more particularly, to a clutch mechanism used in the trimmer potentiometer.
In a rotary trimmer potentiometer including a worm gear rotatably supported in a housing in engagement with a spur gear, rotatably disposed within the housing, for rotating the spur gear when the worm gear is externally turned, and a wiper carried by the spur gear for rotation together therewith for sweeping the resistance element in contact therewith during the rotation of the spur gear, the provision of a clutch mechanism is required for interrupting the transmission of a rotational force from the worm gear to the spur gear only when the wiper rotating together with the spur gear is brought to one of the opposite ends of the resistance element, while permitting the worm gear to be turned idle without rotating the spur gear further.
The prior art clutch mechanism is disclosed in, for example, U.S. Pat. No. 3,099,810 patented July 30, 1963, some of the drawings of which are herein reproduced as FIGS. 11 and 12 for the purpose of discussion of the prior art considered most pertinent to the present invention.
Referring now to FIGS. 11 and 12, the spur gear 2 employed in the prior art potentiometer and having teeth 3 is formed with a blank base 4 by removing some of the teeth 3. A generally ring-shaped ratchet spring member 5 having a stopper projection 8 and a pair of opposite elastically yieldable clutch pawls 6 and 7 substantially functioning as dummy teeth is positioned on one side of the spur gear 2 with the elastically yieldable clutch pawls 6 and 7 arranged radially outwardly of the spur gear 2 adjacent the blank base 4. The clutch pawls 6 and 7, when not deformed, lie in approximately the same position as normal teeth adjacent the respective end teeth of the spur gear 2 confronting the blank base 4.
The prior art potentiometer utilizing the clutch mechanism of the above described construction is so designed and so operable that, when the worm gear 1 is rotated in one direction shown by the arrow A2 to rotate the spur gear 2 in a direction shown by the arrow B2 and when the wiper rotatable together with the spur gear 2 is subsequently brought to one end of the resistance element with the stopper projection 8 abutting against a stopper boss rigid with the potentiometer housing, the end tooth, identified by 3a, of the spur gear can have just moved beyond the teeth of the worm gear 1 and, therefore, further rotation of the worm gear 1 in the direction A2 can cause the associated clutch pawl 7 to bend from its normal position shown by the phantom line and then spring back to its former position. However, when the worm gear 1 is rotated in a direction A1 counter to the direction A2, a force can be applied from the adjacent tooth of the worm gear 1 to the clutch pawl 7 in a direction required to cause the spur gear 2 to rotate in a direction B1 counter to the direction B2. The clutch pawl 6 functions in a manner similar to the clutch pawl 7 when the wiper is brought to the opposite end of the resistance element as a result of the rotation in the direction B1.
The prior art potentiometer appears to be costly in view of the fact that the ratchet spring member which is separate from the spur gear is employed and, therefore, the method of fabricating it is relatively complicated. Moreover, there is a relatively great possibility that the ratchet spring member will not be properly positioned relative to the spur gear, and once the ratchet spring member fails to be properly positioned, the potentiometer as a whole will be rejected.